The present invention relates to an improved method of making a MgO-Containing potassium salt fertilizer using a langbeinite slurry binder and, more particularly, to an improved method of making the aqueous langbeinite slurry binder used in the granulation of MgO-containing fertilizers.
Methods of making a MgO-Containing potassium salt fertilizer using langbeinite (K.sub.2 SO.sub.4.2MgSO.sub.4) as a binder are known. For example, U.S. Pat. No. 3,926,609 to Effmert et al describes a method of making a high abrasion resistant and compression resistant fertilizer by mixing dry, hot potassium chloride or sulfate or a mixture thereof, optionally in combination with kieserite, with hot moist langbeinite at a final mixing temperature of from about 75.degree. to 110.degree. C. and a total moisture content of 3 to 10 % and cooling the mixture with agitation to under 60.degree. C. Fines are recycled so that a grain size from 1 to 4 mm is obtained.
It is the use of langbeinite as a binder which produces fertilizer granules which are particularly pressure and abrasion resistant so that they can be handled practically without generation of dust, which can otherwise be considerable leading to many problems as described in German Patent 23 16 701.
Langbeinite is used as a fertilizer binder conventionally in solid form and is obtained by evaporation of the liquor from K.sub.2 SO.sub.4 production after filtration and washing to free it from MgCl.sub.2. In more detail, the solid langbeinite used previously may be produced as shown in the process in FIG. 1. Leonite(after filtration) and potassium chloride are reacted in water to form potassium sulfate in step B. The liquor formed in the potassium sulfate reaction in step B is reacted in step A with kieserite (MgSO.sub.4.H.sub.2 O) to form a potassium magnesia liquor and the leonite which is used in step B. The potassium magnesia liquor from step A is then evaporated and the concentrate is filtered to produce the solid langbeinite (after washing).
The production of langbeinite by crystallization from an aqueous solution of 2 mol magnesium sulfate and 1 mol potassium sulfate at a temperature above 85.degree. C. has been described in "Gmelins Handbuch der Organischen Chemie", 8.Auflage, Systen-Nr.22, Kalium-Anhangband (1942), page 101f. However, even at such a high temperature it has been shown that it is difficult to separate the product from concentrated mother liquor.
The problem with using langbeinite in solid form as a fertilizer binder is that it "ages" so that it becomes unsuitable for use as a binder for granulation of MgO-containing fertilizers after some time.